you are very light on lands for having a 6 mana commander. I would say 38 minimum. I would cut your mana artifacts to make room. You will also want to prioritize jumping from 4 to 6 mana. Think about Explosive Vegetation, Market Festival, or Circuitous Route

With shelob being such a high cmc I would recommend some Undying Evil-style cards. Maybe Not Dead After All, or Undying Malice.

I'm a huge Nyx Weaver fan and it could be cool to have some graveyard synergies as well since you're in the perfect colors for it. Your card draw package is a bit weak and I'm worried about you running out of gas so I would add something like Moonlight Bargain or Harmonize with some Regrowth effects. I also love Arachnus Spinner and Arachnus Web as a removal pair, and Halana, Kessig Ranger is the creme de la creme of pissing off your opponents with everything having deathtouch.

this deck sucks

Meren as a commander really pushes you to build with a high density of creatures-which are currently only a third of your deck. In this case I would cut most of your non-creature ramp for cheap mana dorks (Somberwald Sage, Arbor Elf, Fyndhorn Elves, maybe even Viridian Emissary, Solemn Simulacrum or Springbloom Druid).

I would also do the same with your other critical deck areas such as removal, draw, boardwipes, sac outlets. I would cut Alesha's Legacy, Overwhelming Remorse, and Putrefy in favor of Merciless Executioner, Insidious Fungus, Grist, the Hunger Tide, Foundation Breaker, and Acidic Slime. Also with more creatures gives us better tutor options: Fauna Shaman is worth the money here, and Jarad's Orders is super efficient. Mausoleum Secrets is kinda cool but it only grabs black creatures which I don't like- up to you if you want to keep it.

Drawing cards is overrated in this deck- your hand IS your graveyard. I would lose the Village Rites, Read the Bones, Sign in Blood, Bastion of Remembrance and Disciple of Bolas to bring in cards like Golgari Thug, Stitcher's Supplier, Skull Prophet, or Satyr Wayfinder. Nyx Weaver and Golgari Grave-Troll are an absolute must.

Not sure terastadon belongs here, as much as I love it. It's nice as a reanimation target but meren will have trouble bringing it back and the deck isn't ramp heavy enough to reliably have the mana to cast it. Instead, maybe a Skullwinder or Junji, the Midnight Sky to give you some added recursion.

You may want to also add in about 5 cheap nonbasics. Temple of Malady, Woodland Cemetery, Dakmor Salvage, Tainted Wood, Twilight Mire, Viridescent Bog, Necroblossom Snarl are all very solid choices. Lands entering tapped won't be a super large issue if you stick to 5 or so but you will definitely notice the mana smoothing and appreciate it.

Hope this helps!